Lesson 10: Weather and Climate — Tropical Singapore

Singapore sits one degree north of the equator, and its weather shows it: hot, humid, and punctuated by sudden downpours all year round. Weather is the day-to-day state of the atmosphere — temperature, rainfall, humidity, wind — while climate is the average weather pattern over at least 30 years. This lesson reads the data of our tropical climate and explains the monsoons and storms that shape daily life here.

Singapore's climate data

Typical figures (long-term averages): daily maximum temperature about 31 °C, minimum about 24 °C; relative humidity averages 80–85%; annual rainfall about 2300 mm spread over roughly 170 rain days. Monthly rainfall for a representative year:

Total annual rainfall = 2318 mm; monthly mean = 2318 / 12 ≈ 193 mm. Notice the pattern: November–January are wettest; February and mid-year months are relatively drier, though no month is truly dry — Singapore has a tropical rainforest climate (Af in the Köppen classification) with rain in every month.

The monsoons

Winds reverse with the seasons because land and sea heat up at different rates. The Northeast Monsoon (roughly December to March) brings moist air from the South China Sea — the wettest period, with monsoon surges bringing days of continuous rain and noticeably cooler temperatures, sometimes falling to around 21–22 °C. The Southwest Monsoon (roughly June to September) is drier for Singapore because the air crosses Sumatra, whose mountains strip much of its moisture; these are also the months when transboundary haze may arrive if fires burn in the region. Between monsoons, inter-monsoon periods bring light variable winds and intense afternoon thunderstorms — including Sumatra squalls, fast-moving lines of thunderstorms that roll in from the Strait of Malacca, often before dawn or in the early morning.

Why so hot, and why the rain falls so hard

The Sun is nearly overhead year-round at the equator, so solar energy arrives concentrated (per square metre) and days are about 12 hours long all year — little seasonal variation. Strong heating warms the surface air, which rises (becomes less dense), cools as it rises, and its water vapour condenses into towering cumulonimbus clouds. The latent heat released during condensation fuels further rising — the engine of our violent but short afternoon storms. Relative humidity = (actual water vapour in air / maximum the air can hold at that temperature) × 100%. Warm air holds more vapour, so at 31 °C and 85% relative humidity the air carries a great deal of water — and cooling by even a few degrees, as in air-conditioning or rising air, pushes it to condensation.

Practical focus: reading a rain gauge and thermometer

A rain gauge collects rain in a funnel feeding a measuring cylinder; depth is read in millimetres at the bottom of the meniscus, at eye level, at a fixed time daily. Siting matters: an open area away from buildings and trees, rim at standard height, otherwise overhangs shield the gauge and readings are systematically low. Maximum–minimum thermometers in a Stevenson screen (white, louvred, raised) measure air temperature in shade with free airflow — direct sun on a thermometer measures the thermometer, not the air.

Singapore focus: floods, heat and forecasting

The Meteorological Service Singapore issues heavy-rain and flood warnings; PUB has widened canals and built detention tanks after major floods (Orchard Road, 2010–2011) to handle intense bursts. The Heat Stress advisory uses the Wet Bulb Globe Temperature: above certain levels, outdoor activities are curtailed because high humidity blocks sweat evaporation — your body's cooling fails exactly when evaporation cannot keep up, linking weather data directly to the particle physics of Lessons 5 and 9.

Common mistakes

• Calling one hot week "climate change". Climate is the long-term average; a single event is weather. Climate claims need decades of data.
• Saying humidity measures "how much rain there is". Humidity measures water vapour content of air, not rainfall.
• Stating the equator is hot "because it is nearer the Sun". The distance difference is negligible; the Sun's rays strike vertically and are concentrated on a smaller area, and travel through less atmosphere.
• Reading a rain gauge from above the meniscus or reporting rainfall in cm³ — rainfall is a depth (mm), not a volume.

Exam Experience

Question 1 (structured, data-based). Use the rainfall table in this lesson. (a) Calculate the total rainfall for the three wettest consecutive months. [2] (b) Describe the rainfall pattern over the year. [3] (c) Explain why the wettest months coincide with the Northeast Monsoon. [2]

Question 2. Which statement best explains why Singapore experiences small seasonal temperature variation? [1] A. The sea around Singapore stores heat and releases it at night. B. The Sun is high in the sky throughout the year, so insolation per unit area stays high and steady. C. Rain falls in every month, cooling the air equally. D. Singapore's buildings trap heat in every season.

Question 3. On a still morning, fog forms over the Singapore River but clears by 9 a.m. Explain the formation and the clearing of the fog in terms of water vapour and temperature. [4]

Marking scheme. Q1(a) Wettest three consecutive: Nov 266 + Dec 309 + Jan 244 = 819 mm [1 correct months identified, 1 total]. Q1(b) Rainfall occurs in all months / no dry season [1]; peaks in November–January (266–309 mm) [1]; minimum around June–August (145–161 mm) but still substantial [1]. Q1(c) During the NE monsoon, winds blow from the South China Sea towards Singapore [1]; they pick up moisture over the warm sea and release it as heavy rain on arrival / rising over land [1]. Q2 B [1]. Q3 Overnight the ground and air cool [1]; air reaches dew point and water vapour condenses onto tiny particles as droplets — fog [1]; after sunrise the Sun warms the air [1]; droplets evaporate as the air can hold more vapour / relative humidity falls, so fog clears [1].

Going deeper: averages, anomalies and sea breezes

A week of daily maximum temperatures at a school station reads: 31, 30, 32, 29, 31 °C... take the five-day set {31, 30, 32, 29, 31}: the mean is (31+30+32+29+31)/5 = 153/5 = 30.6 °C. A single cooler day of 26 °C during a monsoon surge would drag a mean down noticeably — which is why climate scientists use 30-year normals: averages long enough that individual surges and heatwaves wash out. When a data question gives you five years of rainfall and asks about "climate", be suspicious — that is weather data wearing climate's clothes, and saying so, with the 30-year reason, is a distinction-level comment.

The sea breeze, mechanistically. Land has a lower specific heat capacity effect in practice — surfaces heat fast under the morning Sun; the sea warms slowly. By early afternoon the air over land is warmer, less dense and rising; cooler, denser air slides in from the sea to replace it: the onshore sea breeze that makes East Coast Park pleasant at 3 p.m. At night the land cools faster than the sea, the circulation reverses, and a gentle land breeze blows offshore. Singapore's planners use this: building corridors aligned to channel breezes (and monsoon winds) into estates reduce heat-island intensity — the urban heat island effect, in which concrete and tarmac keep city districts up to 7 °C warmer than rural areas at night, is itself a data pattern (city vs rural minimum temperatures) that exam questions mine.

Dew point, one more time. Air at 30 °C and 80% relative humidity needs only a small temperature fall to saturate. Radiative cooling on clear nights drops surfaces below the dew point → dew and the river fog of Lesson 10's exam. On cloudy nights the cloud blanket reduces cooling — less fog. Cause chains like this are the currency of weather questions.

Exam-style extra

Question. A Stevenson screen at a school records maximum temperatures over five days: 33, 32, 34, 26, 33 °C. (a) Calculate the mean maximum temperature. [2] (b) The 26 °C day coincided with a monsoon surge. Explain why including or excluding that day changes conclusions about the week's "typical" weather, and state which figure — mean or median — better resists such an anomaly and its value here. [3]

Marking points. (a) (33+32+34+26+33)/5 = 158/5 = 31.6 °C [1 method, 1 answer]. (b) The surge day is atypical (an anomaly), pulling the mean below the usual value [1]; the median (33 °C) is unaffected by one extreme value [1]; value 33 °C [1].

Workshop: heat index and ultraviolet — weather that touches skin

Temperature alone undersells tropical danger. The heat index combines temperature with humidity: at 31 °C and 80% relative humidity, the air FEELS like about 36–38 °C to a sweating human, because evaporation (Lesson 5) slows. NEA issues heat-stress advisories on this combined measure; schools postpone outdoor activity when it spikes. Any exam answer about "feels hotter" that ignores evaporation is incomplete.

Ultraviolet is the invisible counterpart: Singapore's UV index at midday commonly reaches 11+ ("extreme") because the Sun is near-overhead and the tropical atmosphere is thin and clear. UV damages skin and eyes cumulatively; shade, hats and sunscreen are dose-reduction strategies, and the dose multiplies with reflection off water and pale concrete. The 30-30 lightning rule completes the safety trio: if the gap between flash and thunder is under 30 seconds, seek shelter; wait 30 minutes after the last thunder. Weather questions at G3 level live where physics meets these decisions — evaporation, radiation, and electrical discharge, each with a measurable number attached.